When the jails fill up
Back then they were blaming the overcrowding conditions on state prison policy.
But the State of Texas has fixed their prison population problems (to some extent) and Harris County is still with overcrowded jails. Much of the jail population is detainees -- people who have not been convicted of a crime but just can't make bail. Judges could move to reduce bail requirements to relieve overcrowding.
But that would be easy. And cheap. Better to just contract with out of state jails to house them. That way they are far enough away from friends and family so that we can be sure they won't be able to make bail. Gives them incentive to plead guilty to save the county the trouble of actual trials.